I have six cats.. I find that the siblings are the closest.. one of my
females gets along with everyone, the other is territorial.. same with the
boys.. some are laid back, some are testy.  All were neutered before 6
months of age, most were done at 15 weeks.

However at that young of an age they are pretty much gender neutral.  And as
long as your cat is tolerant, it should be fine.

But just so you know, six weeks is WAY too young for the kitten to be away
from its mother.  If there is a mother and siblings it is best if they stay
together until they are AT LEAST eight weeks, but 12 to 16 is better.. There
is a lot of socialization that goes on, manners are taught, along with
proper play behavior.  Kittens that are taken away too young are obnoxious
because they aren't taught control, they tend to be biters and are generally
very unruly, and chances are it will tick your resident cat off and they'll
never bond.

If the current owner refuses to keep them any longer, or there is no mother,
there is training that you yourself can do, but I would recommend you pick
up an orphaned kitten book for help with that.  If you don't, you are just
asking for trouble.